In order to manufacture advanced materials such as semiconductor devices, trace impurities on the parts per million (ppm) to parts per billion level (ppb) and even sub-ppb levels in any of the manufacturing constituents must be substantially avoided. For example, in manufacturing semiconductor devices, impurities (e.g., moisture, oxygen and organic compounds) in process gases adsorb on the semiconductor wafer, degrade its performance, and lower the manufacturing yield and device reliability.
There are generally two types of impurities in process gases: heterogeneous (e.g., particles and macromolecules) and homogeneous (e.g., dissolved particles). Typically, heterogeneous impurities are removed by filtration and homogeneous impurities are removed by purification. There has been particular interest in the removal of homogeneous impurities from process gases.
There are various commercial purification techniques and purifiers which work by physical adsorption of impurities or by catalytic conversion of impurities to other forms which can be adsorbed. Another exemplary purification technique is a packed bed of particles of a gatter material. Exemplary getter materials include various resins (e.g., Nanochem.RTM. resins) and various alloys (e.g., Zr-V-Fe alloys). For purification, the gas stream passes through these packed beds and the impurities react with the getter material. Although these membranes filter on the ppm level, the membranes often do not effectively filter trace homogeneous impurities on the ppb level. Moreover, these membranes tend to be ineffective when there is an abrupt surge in the impurity level due to the inefficiency of the membranes in bringing impurity molecules in contact with the resins or alloy. Additionally, these membranes tend to generate heterogeneous impurities because of mechanical motion and attrition of particles of the getter material. These membranes also are typically not reusable and often cannot be regenerated.
Additionally, the use of porous ceramics for filtration of particles are known. For example, U.K. Patent No. 2,021,355 to Dahlquist et al proposes a porous membrane for separating heterogeneous impurities from an aqueous medium. The porous membrane comprises an outer support matrix having through-passages and an inner layer lining the through-passages and deposited on the outer support. The support matrix can be a polymer, metal or ceramic. The inner layer can be a matrix of particles of aluminum hydroxide, partially hydrated aluminum oxide, silicon dioxide or zirconium dioxide. French Patent No. 2,251,351 proposes a microporous ceramic filter comprising a microporous ceramic support electrophorethically coated with an oxide of Al, Si, Mg, Ti, Cr, Ni, Zr or Fe. U.S. Pat. No. 3,288,615 to Estes et al proposes a ceramic filter body comprising a framework of one or more tectosilicates with a mineral species (e.g., aluminates and oxides) distributed throughout and filing the framework.